This invention relates to electromechanical relays and more particularly to such relays employing piezoelectric laminates as active elements.
Piezoelectric relays have in recent years shown promise as alternatives for relays operated electromagnetically. In addition to not requiring windings and cores, such relays offer a number of other advantages among which may be mentioned their low power consumption and heat generation, reduced physical size, relatively simple component parts, and, importantly, their potential for batch fabrication by printed wiring techniques. Further, the voltages required for their operation are sufficiently low to permit integrated circuit control.
Typically, the switch element of a relay operated by piezoelectric or electrorestrictive effect comprises a laminate formed of two layers of piezoelectric ceramic material each having an electrode coating fired to each side. The two coated sheets are cemented to opposite sides of a separating conductive centervane which centervane, in one mode of operation, also constitutes one electrode of the relay. In a well-known fabrication step, the piezoelectric material of each layer has a remanant polarization induced therein by applied D.C. electric fields. For the parallel mode of operation contemplated, the layers are polarized in the same direction. In one prior art arrangement, the piezoelectric laminate is mounted at one end on a base member and spaced therefrom by a spacer block. A bracket also mounted on the base member at its other end carries a contact spaced apart from and in alignment with a contact carried at the free end of the laminate. Flexure of the laminate to close the contacts is accomplished in the parallel mode by connecting and grounding the outer electrode coatings of the two layers and applying an operating voltage to the centervane. As a result, electrostatic fields are created in the layers which in one layer agree with the direction of polarization and in the other layer oppose that direction. In accordance with electrostrictive phenomenon of piezoelectric materials, one layer expands lengthwise while the other layer contracts. The resulting stresses cause the laminate to bend; for the cantilever laminate here envisioned, the bending motion is perpendicular to the planes of the laminate electrode coatings thereby causing the contacts to close. Removal of the operating voltage restores the contacts as a result of the restoring mechanical effect of the remanent polarization of the piezoelectric layers.
Although piezoelectric relays have proved themselves in many applications, their manufacture has been attended by a number of problems. Thus, for example, the forces and deflections attainable by electrostrictive effect are relatively small with the result that manufacturing and assembly tolerances may frequently be critical. Further, the difficulty of achieving flat piezoelectric laminates, or bimorphs as they are frequently termed, that is, laminates of uniform surface contours, presents the problem of obtaining consistent contact separation and closure force. A more efficient utilization of the electrostrictive effect as well as less stringent requirements for contact gap control during fabrication, would thus simplify the manufacture of piezoelectric relays and thereby enhance their attractiveness as circuit control elements and it is with these considerations that this invention is chiefly concerned.
It is accordingly an object of this invention to improve the efficiency of piezoelectric relays.
Another object of this invention is to simplify the manufacture of piezoelectric relays.
Also an object of this invention is to relieve manufacturing margins and tolerances in the manufacture and assembly of piezoelectric relays.
A further object of this invention is to provide an improved piezoelectric relay construction which readily lends itself to automatic assembly.